1. Field of the Invention
The present invention relates to a piston used in a hydraulic clutch of an automatic transmission of a vehicle.
2. Description of the Conventional Art
A hydraulic clutch in an automatic transmission of a vehicle is structured such that a piston (a clutch piston) moving in an axial direction within a cylinder by a hydraulic pressure transmits a power from a drive shaft to a driven shaft by bringing a drive plate in the drive shaft side in a multiple disc clutch into pressure contact with a driven plate in the driven shaft side, and shuts off the power transmission by canceling the pressure contact state. Further, this kind of clutch piston generally employs a seal integral type piston (also called as a bonded piston seal) in which a seal portion made of a rubber-like elastic material and brought into close contact with an inner surface of a clutch cylinder is integrally formed in a piston main body.
Since this kind of automatic transmission tends to have multiple stages in recent years, and a front-wheel drive (FF) system has a constraint on space with respect to an axial direction, the clutch piston tends to be enlarged in diameter particularly in the automatic transmission having a great capacity. However, in the case that the clutch piston is constituted by the seal integral type piston as mentioned above, a metal mold apparatus or the like for integrally forming a seal portion in the piston main body becomes large in size if it is intended to manufacture a seal integral type piston having a large diameter. An increase of a manufacturing cost is unavoidable.
Accordingly, there has been conventionally developed a technique of manufacturing a seal ring as an independent member from a piston main body and fitting the seal ring in a close contact manner to the piston main body (refer to Japanese Unexamined Patent Publication No. 2003-139249 and Japanese Unexamined Patent Publication No. 2006-2915).
However, in Japanese Unexamined Patent Publication No. 2003-139249 of the conventional arts, since a thin portion is formed in a fitting portion to a cored bar of a seal ring in a piston main body, and a step portion for providing a lip sealing between the cored bar and the piston main body is formed in an inner peripheral portion of the cored bar, a working cost becomes high. Further, in Japanese Unexamined Patent Publication No. 2006-2915, since a similar step portion is formed in an inner peripheral portion of a cored bar of a seal ring, there is a problem that a working cost becomes high. Further, since a fitting portion between the cored bar and the piston main body is sealed by only one stage, Japanese Unexamined Patent Publication No. 2006-2915 has trouble with a sealing performance thereof.